1. Technical Field
The invention relates generally to signal processing and more particularly, to space-borne signal processing for use at low temperatures and in a radiation environment.
2. Background Art
Space based threat warning systems use infrared focal plane arrays to detect potential missile threats. However, infrared focal plane arrays (FPAs) which operate in space have two inherent limitations. They must function in a cryogenically cooled environment (i.e. in a dewar) and their output must be corrected to compensate for errors induced by incident gamma rays. Currently, errors caused by gamma radiation, known as gamma spikes, are corrected using analog processing in a dewar or by digital or analog processing outside the dewar. Using analog processing inside the dewar or digital processing outside the dewar increases the amount of noise introduced into the FPA data. A typical focal plane signal processor is shown in FIG. 13. FIG. 13 shows a typical infrared missile warning system with a staring sensor. A lens 12 is used to focus the IR energy from a portion of the sky/ground scene through an optical filter 14 and onto a focal plane array 16. The lens typically has a 60.degree..times.60.degree. field of view (FOV) and allows the sensor to collect IR energy from or "stare at" a portion of the sky/ground scene. This type of system is known as a staring system. Another category of infrared missile warning system uses a scanning sensor. In the scanning system the sensor is a line array (rather than a rectangular FPA in a staring system) and the optics scan the scene through the line array.
The filter 14 can be a color wheel. Alternatively, the color wheel may be replaced with an acousto-optic tunable filter (AOTF). The purpose of an optical filter is to limit the thermal energy which passes to the FPA. Targets have signatures with large amounts of thermal energy in very specific frequency bands. The optical filters are tuned to allow only energy in the target specific frequency bands to pass. Therefore, targets imaged by the FPA will have a greater signal to noise ratio after optical filtering.
The focal point array 16 is a matrix of individual detector elements or pixels, which emit electrons when struck by incident photons. The term pixel is an abbreviation for picture element and is the smallest uniquely definable element in the scene. The value of each pixel represents the amount of infrared energy incident on a detector. The electrons emitted by the detectors are captured beneath each pixel and the quantity of electrons captured is directly proportional to the intensity of the objects within the immediate field of view (IFOV) of the pixel. A typical focal plane array is composed of 16,384 pixels arranged as a matrix of 128 by 128 elements. The data from all pixels in the array can be read as one snapshot of the scene. This snapshot is called a frame of IR data.
The signals from the pixels of the FPA 16 are amplified by amplifier 18 and converted to a digital form in A/D converter 20. The data are then processed to compensate for nonuniformities in the FPA which were introduced when the array was manufactured. The compensated data are spatially filtered to increase the signal to noise level of potential targets in the data. After spatial filtering, the data may be spectrally filtered and detected. All this is done in signal processor 22. Any detected targets are sent to the data processor for further processing. An inertial navigation system (INS) 26 outputs data to the data processor 24 to aid in tracking any detected threats.
What is needed is a focal plane processor having low noise with correction for gamma errors and able to operate at or below liquid nitrogen temperatures and be radiation hardened. This can be done by digitally processing the FPA data inside the dewar to reduce the noise and provide added resolution. The added resolution yields significantly better target detection and tracking performance.